Optical fiber configuration for dissipating stray light

ABSTRACT

An optical transmission fiber is formed to include a relatively low-index, relatively thin outer cladding layer disposed underneath the protective polymer outer coating. Stray light propagating along an inner cladding layer(s) within the fiber will be refracted into the thin outer cladding (by proper selection of refractive index values). The thin dimension of the outer cladding layer allows for the stray light to “leak” into the outer coating in a controlled, gradual manner so as to minimize heating of the coating associated with the presence of stray light. The inventive fiber may also be bent to assist in the movement of stray light into the coating.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 11/705,229filed Feb. 12, 2007 now U.S Pat. No. 7,437,046 and issued a Notice ofAllowance on Jul. 3, 2008 and claims the filing date benefit thereof.

TECHNICAL FIELD

The present invention relates to an optical fiber useful for managingthe presence of stray light in fiber-based laser, amplifier or lightcombiner applications and, more particularly, to an optical fiberincluding a thin outer cladding layer disposed between an inner claddingand an outer coating, the thin outer cladding used to contain and manageany light (pump and/or signal) that is present in the inner claddinglayer, and dissipate this stray light in a controlled manner to minimizeheating of the fiber's outer coating.

BACKGROUND OF THE INVENTION

Cladding-pumped fiber devices, such as lasers, amplifiers and lightcombiners, are important in a wide variety of optical applications,including high power communication systems, light sources for printers,lasers for medical optics, and the like. A typical cladding-pumpedoptical fiber comprises a signal core and a plurality of claddinglayers. The inner cladding surrounding the core is typically a silicacladding of large cross-sectional area (as compared to the core) andhigh numerical aperture (NA). It is usually non-circular to ensure thatthe modes of the inner cladding will exhibit good overlap with the core.An outer coating is commonly composed of a low index polymer. The indexof the core is greater than that of the inner cladding which, in turn,is greater than the index of the outer coating.

A major advantage of the cladding-pumped fiber is that it can convertlight from low brightness sources into light of high brightness in thesingle mode fiber core. Light from low brightness sources, such as diodearrays, can be coupled into the inner cladding as a result of its largecross-sectional area and high numerical aperture. In a cladding-pumpedlaser or amplifier, the core is doped with a rare earth such asytterbium (Yb) or erbium (Er). The light in the cladding interacts withthe core and is absorbed by the rare earth dopant. If an optical signalis passed through the pumped core, it will be amplified. Alternatively,if optical feedback is provided (as with a Bragg grating opticalcavity), the cladding-pumped fiber will act as a laser oscillator at thefeedback wavelength.

FIG. 1 illustrates an exemplary prior art cladding-pumped fiber 1 havinga core 2, an inner (or pump) multimode cladding layer 3, and an outercoating 4. Inner cladding layer 3 exhibits a refractive index lower thanthat of core 2 such that the light signal L propagating along core 2will remain confined therein, as shown in FIG. 1. Similarly, outercoating 4 confines pumping light P within the boundaries of innercladding layer 3, as shown. In accordance with the cladding-pumpedarrangement, the rays comprising pump light P periodically intersectcore 2 for absorption by the active material therein, so as to generateor amplify light signal L. It is to be noted that since inner cladding 3is multimode, many rays other than those shown by the arrows in FIG. 1can propagate within inner cladding 3.

A difficulty preventing full exploitation of the potential ofcladding-pumped fiber devices is the problem of efficiently coupling asufficient number of low brightness sources into the inner cladding. Aproposed solution to this problem is described in U.S. Pat. No.5,864,644, entitled “Tapered Fiber Bundles for Coupling Light Into andOut of Cladding-Pumped Fiber Devices”, issued to D. J. DiGiovanni et al.on Jan. 26, 1999. In the DiGiovanni et al. arrangement, light is coupledfrom a plurality of sources to a cladding-pumped fiber by the use of atapered fiber bundle, formed by grouping individual fibers into aclose-packed formation and heating the collected fibers to a temperatureat which the bundle can be drawn down into a tapered configuration. Thetaper is then fusion spliced to the cladding-pumped fiber. FIG. 2illustrates an exemplary embodiment of this DiGiovanni et al. prior artapproach, where a plurality of pump fibers 5 are shown as distributedaround a fiber containing a core 6. As shown, the entire bundle 7 isfused and tapered along a section 8 to a single output cladding-pumpedfiber 9. As described therein, tapering of the fiber bundle is performedto increase the intensity of pump light coupled into the end ofcladding-pumped fiber 9. Inasmuch as the NA of the multimode pump regionis much greater than the NA of the pump fibers, tapering of the fiberbundle allows for an increase in the optical pump intensity whileremaining within the angular acceptance of the multimode pump region.

Even though the DiGiovanni et al. tapered fiber bundle has been found togreatly improve the efficiency of coupling multiple optical signals intoa fiber amplifier, laser or light combiner, problems attributed to thepresence of “stray light” within the system remain to be solved. Straylight has been found to arise from a number of different sources, suchas amplified spontaneous emission (ASE) within a gain fiber, unabsorbedor scattered pump light, and signal light that has scattered out of thecore and into the inner cladding. While the prior art arrangement ofFIG. 1 is capable of transmitting stray light with minimal attenuationand without heating the fiber, stray light may result in catastrophicheating if it is not permanently contained within the boundary of innercladding 3. The escape of stray light from the cladding can occur if theNA of the cladding light is increased at a perturbation (such as ataper) to exceed the NA between inner cladding 3 and outer coating 4. Inthis situation, cladding light refracts into outer coating 4 where it isabsorbed and generates an unwanted amount of localized heating. Straylight may also refract into outer coating 4 at a termination of thecladding-pumped fiber, such as at the point where it is spliced to anoutput fiber (such fibers generally have a high index outer coating) orat any point along the fiber where it is bent to a degree sufficient tocouple light into the cladding layer.

FIG. 3 illustrates the above-described situation where stray light isassociated with a termination condition, in this case at a splice Sbetween cladding-pumped fiber 1 of FIG. 1 and an output fiber 11. Asshown, unabsorbed/scattered pump light remaining at the termination ofcladding-pumped fiber 1 enters output fiber 11 and refracts into a highindex polymer outer coating 13. Since the optical absorption of polymerouter coating 13 is much greater than that of glass, a significantportion of the light is absorbed by coating 13 and converted to heat. Ifthis heat is sufficiently localized, the fiber may be burned orotherwise damaged to the point of experiencing catastrophic failure.Besides the presence of unabsorbed pump light, signal light can also bescattered out of core region 2 at the termination of fiber 1, whereuponit will propagate along inner cladding 15 and may also refract into highindex cladding 13 to cause additional heating.

While heating can arise at a splice location between two dissimilarfibers (as shown here in FIG. 3), splices between identical fibers mayalso generate heat, as a result of slight imperfections that cause lightscattering. Various other types of perturbations along the fiber mayalso result in increasing the presence of stray light along the fiberand thus potentially compound the problem of locally heating the fiber.Since the optical power levels can be high in amplifier applications, itis best to gradually dissipate the energy, thus avoiding localizedheating of the fiber or any of its associated optical components.

Prior art attempts to address this problem typically involve the use ofsections of “absorbing” fiber interspersed along the transmission path,where these sections include selectively absorbing species, such as rareearth ions, in concentrations sufficient to provide the desiredabsorbance selectivity. U.S. Pat. No. 6,574,406 issued to B. J. Ainslieet al. on Jun. 3, 2004, and US Application 2004/0175086 by L. A. Reithet al. and published on Sep. 9, 2004, disclose two differentarrangements of this principle.

While these arrangements provide a certain degree of stray lightmanagement, the utilization of selected sections of fiber to providethis ability limits its usefulness. For example, if a new splice isadded to a fiber, or a bend is introduced in a new location, theabsorbing fiber sections may not be properly located to dissipateadditional stray light. Moreover, the fiber section dimensions need tobe carefully controlled to ensure that the energy is dissipated in asufficiently gradual manner.

Thus, a need remains in the art for a configuration that is capable ofmanaging the presence of stray light within an optical fiber so as tominimize heating of the fiber and/or other failure modes attributed tothe presence of stray light.

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the presentinvention, which relates to an optical fiber configured to controllablydissipate stray light and, more particularly, to the inclusion of thinouter cladding layer between the inner cladding and the fiber outer(polymer) coating to contain light refracting out of the inner claddingand dissipate the light in a controlled manner along an extended portionof the fiber's outer coating.

In accordance with the present invention, an optical transmission fiberis formed to include a relatively thin outer cladding layer disposed tosurround the inner cladding layer and thus capture and contain straylight (including remaining pump light and/or refracted signal light).The limited thickness of the outer cladding permits stray light topropagating therealong while “leaking” or “tunneling” into the outercoating in a controlled manner. In a preferred embodiment, a thicknessof no more than 10 μm (or, even better, 5 μm) is defined for the outercoating layer. By forcing the stray light to be dissipated along anextended portion of the outer coating, localized heating of the polymerouter coating will be virtually eliminated, preventing thermally-inducedcatastrophic failure of the fiber.

The thin outer cladding layer is formed to exhibit a refractive indexless than that of the inner cladding (in order to promote the reflectionof light within the inner cladding), where the outer cladding layer mayexhibit either a step-index or graded-index profile with respect to therefractive index values of the inner cladding and outer coating.

In one embodiment, a plurality of scattering or absorbing sites may beformed within the outer cladding layer, or at the boundary between theinner and outer claddings, to facilitate the movement of stray lightfrom the inner cladding to the outer cladding.

It is an aspect of the present invention that the inclusion of a thin(“leaky”) outer cladding layer may be utilized in virtually anyfiber-based arrangement where thermal management of stray light is aconcern. For example, fiber amplifiers, fiber-based lasers, lasercombiner bundles, all generate a significant amount of stray lightenergy that can become problematic. Further, environmental situations(such as where a fiber needs to be confined in a bent position, or at asplice between different fiber sections) can increase the presence ofstray light. In any of these situations, the inclusion of a thin outercladding layer adjacent to a polymer-based fiber outer coating willcontrollably manage the dissipation of the stray light along an extendedportion of the outer coating.

These and other embodiments and features of the present invention willbecome apparent during the course of the following discussion and byreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings:

FIG. 1 is a side view of an exemplary prior art cladding-pumped opticalfiber;

FIG. 2 is a side view of an exemplary prior art tapered fiber bundleinput to a cladding-pumped optical fiber;

FIG. 3 is a side view of a splice location between a cladding-pumpedfiber and a transmission fiber, illustrating the potential for thecreation of stray light at the splice;

FIG. 4 contains a somewhat more detailed illustration of a prior arttapered fiber bundle, illustrating the introduction ofbackward-propagating stray light into the fiber bundle;

FIG. 5 is an optical/thermal photograph of the prior art tapered bundleof FIG. 4, illustrating the generation of localized heating of thesignal fiber as a result of the presence of stray light;

FIG. 6 is a side view of an exemplary tapered fiber bundle formed inaccordance with the present invention, including “thermally-managedoptical transmission fiber” comprising a thin outer cladding layerwithin the signal fiber to controllably dissipate stray light along theouter coating;

FIG. 7 is a cross-sectional view of an exemplary thermally-managedoptical transmission fiber formed in accordance with the presentinvention;

FIG. 8 illustrates the refractive index profile for the inventive fiberof FIG. 7;

FIG. 9 is a graph of optical loss as a function of fiber bend radius,illustrating the use of controlled fiber bending to assist in theremoval and dissipation of stray light;

FIG. 10 is a refractive index profile of an exemplary inventive fiberused to collect the data for the graph of FIG. 9;

FIG. 11 is a side view of an exemplary laser combiner formed inaccordance with the present invention, with each fiber including a thinouter cladding layer to dissipate stray light; and

FIG. 12 is a cut-away view of the splice location between thelaser-propagating fibers of FIG. 11 and an output fiber, illustratingthe significant amount of interstitial spacing (between fiber cores)that may give rise to the generation of stray light within the system.

DETAILED DESCRIPTION

An exemplary prior art tapered fiber bundle 10 is shown in FIG. 4, inthis case illustrating the propagation of backward-scattering straylight that re-enters bundle 10 from a cladding-pumped fiber 12 that isfused to bundle 10. Bundle 10 is illustrated as comprising a pluralityof pump fibers 14 and a signal fiber 16. Using methods well-known in theart, bundle 10 is adiabatically tapered down until its outer diametermatches the outer diameter of cladding-pumped fiber 12 at location F,where the two fibers are then fusion spliced together. Signal fiber 16comprises a core region 17 (which may be single mode or multimode),surrounded by a relatively large diameter (e.g., 125 μm) cladding layer18. Pump fibers 14 comprise a relatively large silica core 13 (e.g., 105μm) and a thin (e.g., 10 μm), low-index cladding layer 15. As discussedabove, the refractive index of cladding layer 18 is less than therefractive index of core region 17 so as to confine the propagatingsignal light to the fiber axis along the core.

It is known that even small amounts of stray light can result in asignificant rise in the temperature of tapered fiber bundle 10, leading(at times) to catastrophic failure. As mentioned above, stray lightarises from one or more sources, including ASE within signal fiber 16,unabsorbed pump light P associated with a counter-propagating pumpsource (indicated by the “backward” arrow in FIG. 4) and/or signal lightthat scatters out of the core region of signal fiber 16. FIG. 5 containsan optical/thermal photograph illustrating this principle, where thepresence of stray light is induced by the use of a backward-propagatingsignal that is coupled into each of the fibers forming the bundle. Byseparating the fibers and monitoring their temperatures with a thermalcamera, a significantly higher temperature within signal fiber 16 isevident by the white spot within the center of the thermal image.

It has been found that the difference in generated temperature between asignal fiber and pump fibers, such as shown in the photograph of FIG. 5,can be attributed to the particular cladding structure utilized withpump fibers. In particular, and with reference again to FIG. 4, backwardtraveling light that is coupled into a conventional signal fiber 16 willenter the surrounding cladding layer 18, and thereafter be guided intoouter polymer coating 19. Since the polymer has high optical absorption,this light is quickly converted into undesirable heat energy. Lightentering pump fibers 14, on the other hand, is predominantly captured bysilica core 13 and guided at the glass interface between silica core 13and low-index cladding 15. As a result, the backward propagating lightwithin the pump fibers minimally interacts with the overlying polymer,and no significant heating occurs. Therefore, in accordance with thepresent invention, the amount of heating associated with stray lightpropagating along signal fibers is reduced by incorporating anadditional cladding layer to manage the distribution of the opticalenergy along the length of the fiber.

FIG. 6 illustrates a tapered fiber bundle formed in accordance with thepresent invention, where a signal fiber 30 is particularly configured toinclude a thin (i.e., “leaky”), lower index outer cladding layer that isused to strip away the stray light propagating along the inner claddingand controllably leak this stray light along an extended portion of theouter coating. This leaking (or tunneling) effect may be enhanced bybending the fiber, as discussed below. Pump fibers 14 as illustrated inFIG. 6 are essentially identical to those included within the prior artstructure of FIG. 4.

FIG. 7 contains a cross-sectional view of an exemplarythermally-managed, high power signal fiber 30 formed in accordance withthe present invention. As shown in both FIGS. 6 and 7, thermally-managedhigh power signal fiber 30 comprises a core region 32, an inner cladding34 of relatively large cross-sectional area, a thin outer cladding layer36 (where thin outer cladding 36 has a refractive index less than thatof inner cladding 34—either a constant-value refractive index or agraded-index value), and a polymer coating 38 covering outer cladding 36(coating 38 having a refractive index greater than that of innercladding 34). FIG. 8 contains a refractive index profile (not to scale)for the exemplary fiber 30 of this particular embodiment of the presentinvention.

As discussed above, thin outer cladding layer 36 functions to trap andguide any stray light, whether remaining pump light or refracted signallight, and prevent this light from directly interacting with and heatinglocalized portions of polymer coating 38. Since outer cladding layer 36is intentionally formed to be relatively thin (e.g., less than 10microns, or even 5 microns in thickness), the stray light will graduallyleak/tunnel into polymer coating 38 as the light propagates along outercladding layer 36. Indeed, by maintaining the thickness of outercladding 36 to less than 10 μm, stray light will tunnel through outercladding 36 such that the optical energy is thereafter graduallydistributed along an extended portion of outer coating 38.

The tunneling from thin outer cladding 36 into polymer coating 38 can beenhanced by bending the fiber, as mentioned above. In particular, and asshown in the graph of FIG. 9, as the bend diameter of the inventivefiber is reduced, the structure becomes more lossy. The graph of FIG. 9was generated for a fiber having a core diameter of 105 μm, an outercladding diameter of 114 μm, and an outer coating diameter of 250 μm, asshown in the associated refractive index profile of FIG. 10. Thedifference in refractive index between the inner and outer claddinglayers (Δn) was approximately 0.0167, and the outer coating was aconventional UV-cured acrylate coating with an index higher than that ofthe inner cladding. The core was fully filled with light, and thethroughput was monitored at various bend diameters. As shown in FIG. 9,the rate of loss of light can be “tuned” by varying the bend diameter.It is to be noted that even at larger bend diameters it appears that theoptical loss is non-zero. By virtue of the thin dimension of outercladding 36, the bending may be performed without affecting thepropagation of the signal within core region 32.

In most embodiments, the NA between inner cladding 34 and outer cladding36 should be within the range of approximately 0.15-0.33. Using thesevalues, therefore, outer cladding layer 36 may comprise a thickness ofless than 10 μm and provide sufficient bend loss without disturbing thelight signal propagating in core region 32. Outer cladding 36 maycomprise glass or a polymer material. Cladding 36 may also be formed tocontain scattering sites (such as, for example, alumina powder orcrystallized polymer) either within its bulk or at its inner surface, tofacilitate removal of the optical energy from inner cladding 34 anddistribution of the energy along polymer coating 38. Coating 38 may beapplied to the optical fiber during the fabrication process, or may beapplied later, as the fiber is packaged—using a heat sink grease orbonding epoxy in the latter.

While the above discussion has focused on the issue of thermalmanagement within the signal fiber of a tapered fiber bundle, it is tobe understood that similar thermal management concerns are present inother fiber-based optical arrangements where heating due to absorptionof light is a concern. For example, fiber splices and fiber bends areconfigurations that are known to introduce stray light into the system.In these cases, therefore, a similarly constructed high power signalfiber including a thin, low index outer cladding layer may be utilizedto facilitate the removal of this stray light and dissipate the lightalong an extended portion of the outer coating. Indeed, a laser combinerarrangement has been developed where a plurality of fibers that areassociated with separate light sources are combined in a bundle throughtapering and provided, as a group, as an input to a larger-coretransmission fiber. FIG. 11 illustrates one such laser combinerarrangement, including the addition of a thin outer cladding layer alongeach laser input fiber, to provide for thermal management of stray lightin accordance with the present invention.

Referring to FIG. 11, a laser combiner 40 is shown as comprising aplurality of signal fibers 30 (shown as 30-1, 30-2 and 30-3) that arecombined through a tapering arrangement into a large multimode corefiber 42. Each input signal fiber 30 contains high brightness, low NAlight, such as single mode light from a fiber laser. An intendedapplication of such a laser combiner 40 is in association with materialsprocessing, where there is a high likelihood that a significant fractionof light (such as reflections from a molten metal surface) will bereflected back into the bundle of signal fibers as stray light. Uponreaching the entrance of the bundle of fibers 30, some fraction of thestray light will enter the interstitial spaces between the individualcores 32 (see FIG. 12 for an illustration of an exemplary plurality ofcores and extensive interstitial spacing in such a bundle oflaser-propagating fibers) and be guided into surrounding claddingregions 34. Thus, in the same manner as described above, problemsassociated with heating of outer polymer coating 38 are minimized byincluding outer cladding layer 36 to trap the stray light, and graduallydissipate this light along an extended length of polymer coating 38.

Indeed, it is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentsthat can represent applications of the principles of the presentinvention. Numerous and varied other arrangements can be made by thoseskilled in the art without departing from the spirit and scope of thepresent invention as defined by the claims appended hereto.

1. A tapered fiber bundle for combining a plurality of separate opticalsignals and providing the plurality of signals as a single input to anassociated transmission fiber, the tapered fiber bundle comprising: aplurality of fibers for supporting the transmission of a plurality ofoptical signals, at least one fiber supporting the transmission of asubstantially single mode optical input signal, wherein each fibercomprises: a core region for containing and propagating the associatedoptical signal, having an refractive index; an inner cladding layersurrounding said core region and exhibiting a refractive index less thanthe refractive index of said core region; a thin outer cladding layersurrounding said inner cladding layer, said thin outer cladding layerexhibiting a refractive index less than the refractive index of saidinner cladding layer so as to capture any stray light propagatingtherealong, and having a thickness selected to controllably dissipatecaptured stray light away from the optical fiber; and an outer coatingsurrounding said thin outer cladding layer, wherein the stray lightcaptured by the thin outer cladding layer will gradually tunnel into theouter coating over an extended length thereof, reducing localizedheating of said outer coating by the stray light, wherein the outercoating has a refractive index greater than the refractive index of thethin outer cladding layer.
 2. The tapered fiber bundle as defined inclaim 1 wherein the tapered fiber bundle is utilized as an input to acladding-pumped fiber amplifier and the plurality of fibers comprises asingle fiber for supporting transmission of the substantially singlemode optical input signal, the remaining fibers supporting thepropagating of a set of separate pump signals.
 3. The tapered fiberbundle as defined in claim 1 wherein the tapered fiber bundle isutilized as an input to a high power, large core transmission fiber andeach fiber of the plurality of fibers is utilized to support thetransmission of an input laser signal, thereafter combined within thehigh power, large core transmission fiber.